Geochemical and geochronological dataset of rutile from a Variscan metabasite in Sardinia Italy

摘要

A c. 500 m wide and 1.5 km long body consisting of basic to ultrabasic rocks, metamorphosed up to granulite-facies and retrogressed to amphibolite-facies conditions during the Variscan orogeny, crops out near Olbia (NE Sardinia, Italy). Among abundant samples, one, collected from a garnet-rich centimetric layer, was chosen for a detailed analysis of rutile; chemical analyses of rutile were performed with the electron microprobe on petrographic thin sections, whereas U/Pb ages were determined by LA-ICP-MS on rutile mounted in epoxy resin. Chemical analyses show that rutile included in other minerals (Rtinc) commonly show higher SiO2 and FeO contents and lower Nb2O3 and ZrO2 contents if compared with rutile in the matrix of the garnet-rich layer (Rtmat). Cr2O3 concentrations are quite similar in both types of rutile. Rtmat commonly shows a greater variability in minor elements, especially Nb2O3 (0.049–0.284 wt.%) and SiO2 (0.019 - 0.193 wt.%) whereas Rtinc compositions are more homogeneous except for FeO (0.251–0.562 wt.%). The U-Pb isotopic data provided discordant ages and defined a lower intercept in the Tera-Wasserburg diagram of 273 ± 13 Ma. Few compilations of geochemical and geochronological data on rutile in Variscan metabasites can be found in literature, thus these data represent a new insight on a mineral phase the significance and scientific interest of which are rising in the last years. Future studies on the origin and ages of emplacement and metamorphism (either prograde or retrograde) of this kind of rock, widespread in the Variscan chain, will benefit from these data as a term of comparison.